As the telecommunication data traffic grows rapidly, the licensed frequency bands of 3GPP has a tendency of not being able to provide higher network throughput capacity. In order to increase the utilization rate of frequency band resources, it has been discussed for 3GPP to utilize an unlicensed frequency band such as the 2.4 GHz frequency band and 5 GHz frequency band under the help of a licensed frequency band. These unlicensed frequency bands are now mainly used by Wi-Fi, Bluetooth, radar, medical systems, etc.
Under normal circumstances, the access technology designed for a licensed frequency band, such as Long Term Evolution (LTE), is not suitable to be used in an unlicensed frequency band, because the access technology such as LTE has very high requirements for frequency band efficiency and user experience optimization. Whereas, the Carrier Aggregation (CA) function makes it possible for LTE to be deployed in an unlicensed frequency band. The concept of LTE Assisted Access (LAA) is proposed for 3GPP, which can utilize an unlicensed frequency band with the help of a licensed LTE frequency band. There are two working manners of the unlicensed frequency band, one of which is Supplemental Downlink (SDL) which only has downlink transmission sub-frames, and the other of which is the TDD mode which not only has downlink transmission sub-frames but also has uplink transmission sub-frames. The Supplemental Downlink can only be used with the help of Carrier Aggregation technology (as shown in FIG. 1). The TDD mode not only can be used with the help of Dual Connectivity (DC), but also can be used independently.
As compared to a Wi-Fi system, an LTE system working in an unlicensed frequency band is capable of providing higher frequency band efficiency and larger coverage, and in the meantime, based on the same core network, data traffic can be switched seamlessly between the licensed frequency band and the unlicensed frequency band. For users, this means better broadband experience, higher transmission speed, better stability and mobile convenience.
The access technology currently used in an unlicensed frequency band, such as Wi-Fi, has weak anti-interference ability. In order to prevent interference, many interference avoiding regulations have been designed for Wi-Fi systems, such as Carrier Sense Multiple Access/Collision Detection (CSMA/CD). The basic principle of this method is that, before a Wi-Fi Access Point (AP) or a terminal sends signal or data, it is first detected whether there is another AP or another terminal sending/receiving signal or data in the surrounding area, if there is, the detecting is kept on until it is detected that there is not; if there is not, a random number is generated as the avoiding period, and if no signal transmission or data transmission is detected during this avoiding period, then, after this avoiding period is over, the AP or terminal starts to send signal or data. This process is shown in FIG. 2.
But, it is the good orthogonality of the LTE network that guarantees the anti-interference level, so that the uplink and downlink transmission between a base station and a user does not need to consider whether there is another base station or another user transmitting data in the surrounding area. If the use of LTE in an unlicensed frequency band also does not consider whether there is another device using the unlicensed frequency band in the surrounding area, significant interference would be caused to Wi-Fi devices. Because LTE performs transmission whenever there is data traffic without any detecting regulations, the Wi-Fi devices cannot perform transmission when there is data traffic transmitted by LTE, and it is only after the LTE data traffic transmission is completed that the Wi-Fi devices can detect a channel idle state to perform data transmission.
Therefore, when an LTE network utilizes an unlicensed frequency band, one of the major key points is guaranteeing the LAA can coexist with the existing access technology (such as Wi-Fi) on a fair and friendly basis. But there is no Listen Before Talk (LBT) mechanism in the conventional LTE system to prevent collision. For better coexistence with the Wi-Fi system, the LTE system needs an LBT mechanism.
However, the already deployed LBT mechanisms all have a frame based LBT structure, as shown in FIG. 3, wherein the LBT cycle is fixed, and the Clear Channel Assessment period is at the beginning of every cycle. For example, in an LBT frame structure with a 10 ms cycle, the CCA takes one or more symbols at the front of the #0 sub-frame. In such frame structure with a fixed cycle, only the #0 sub-frame can be used for CCA, and if a data traffic arrives in the #1 sub-frame, the detection of whether the channel is available for use must wait until after CCA is performed in the #0 sub-frame of the next cycle, which brings large amount of time delay.
Thus, the technical problem that has to be solved urgently is how to effectively reduce the time delay of data traffic transmission due to channel state detection at a fixed detection period, on the premise that the normal working of the LTE system in the unlicensed frequency band is guaranteed, so as to improve data traffic transmission efficiency and at the same time achieve harmonious coexistence of the LTE system and other systems in the unlicensed frequency band.